On the
Stability of Perfluoroalkyl-Substituted Singlet
Carbenes: A Coupled-Cluster Quantum Chemical Study
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Abstract
A series
of trifluoromethyl-substituted carbenes R–C(:)–CF<sub>3</sub> (R = NMe<sub>2</sub>, OMe, F, PMe<sub>2</sub>, P(NMe<sub>2</sub>)<sub>2</sub>, P(N(Pr-i)<sub>2</sub>)<sub>2</sub>, SMe, Cl);
(dimethylamino)(perfluoroalkyl)carbenes Me<sub>2</sub>N–C(:)–R (R = CF<sub>3</sub>, C<sub>2</sub>F<sub>5</sub>, <i>n</i>-C<sub>3</sub>F<sub>7</sub>, <i>i</i>-C<sub>3</sub>F<sub>7</sub>, and <i>t</i>-C<sub>4</sub>F<sub>9</sub>) and symmetrically substituted carbenes R–C(:)–R
(R = NMe<sub>2</sub>, OMe, F, PMe<sub>2</sub>, SMe, Cl) have been
investigated by means of quantum chemistry methods. Different levels
of approximation were used, including the CCSD(T) approach also known
in quantum chemistry as the “golden standard”, in combination
with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative
stabilities of carbenes have been estimated using the differences
between the singlet and triplet ground state energies (Δ<i>E</i><sub>ST</sub>) and energies of the hydrogenation reaction
for the singlet and triplet ground states of the carbenes. The latter
seem to correlate better with stability of carbenes than the Δ<i>E</i><sub>ST</sub> values. The <sup>13</sup>C NMR chemical shifts
of the methylidene carbon indicate the more high-field chemical shift
values in the known, isolable carbenes compared to the unstable ones.
This is the first report on the expected chemical shifts in the highly
unstable singlet carbenes. Using these criteria, some carbene structures
from the studied series (as, for instance, Me<sub>2</sub>N–C(:)–CF<sub>3</sub>, Me<sub>2</sub>N–C(:)–C<sub>3</sub>F<sub>7</sub>-<i>i</i>) are proposed as good candidates for the experimental
preparation
